Don't panic if you see fall webworms

Friday

Sep 10, 2010 at 2:00 AM

In the last few weeks, many local homeowners have been seeking advice on what to do about tent-like web structures in their trees. They are fearful that it is a tent caterpillar, which can do great damage to trees.

By Carol Oneal

In the last few weeks, many local homeowners have been seeking advice on what to do about tent-like web structures in their trees. They are fearful that it is a tent caterpillar, which can do great damage to trees.

In all likelihood, however, what they are seeing is the fall webworm. The first way to tell the difference between the two is to look at the webbing. Tent caterpillars build their neat-looking webs in tree trunk or branch crotches, while fall webworms have a messier looking web structure out on the limbs.

Tent caterpillars appear in spring, while fall webworms show up in late summer or fall, which is why they are called fall webworms. In our area, the madrone tree is the most likely choice of a home for the webworm, although they also will infest maple, ash, oak, walnut or almost any other hardwood.

The caterpillars, which are inside the web busily eating leaves, vary in color from yellowish to gray, are fuzzy, and about a half-inch long. When they first hatch, worms eat on the leaf surface, but as they grow, they enlarge the web to enclose more leaves. This silky tent could expand to enclose an entire branch.

While all of this sounds kind of scary and makes you feel like you should do something quick to control them, you probably need to do nothing.

First, infestations tend to occur about every five years, often following cool, wet springs. Second, north of 40 latitude — which is where we are — the webworms produce only one generation of offspring a year. Thus, they do not build up huge populations. Third, your tree already has produced and stored energy to survive the upcoming winter, so it is not in physical danger. The only exception to this would be if you had a sapling that was entirely encased with webs.

Generally, however, the major offense of the webworm is in the appearance of the tree. This could be remedied by cutting off the offending branch or the part of it that is affected. But keep in mind that when the caterpillars reach maturity, they will leave the nest, so don't wait too long.

The adult looks like a hairy, white moth; some of them might have black or brown spots on the forewings. The moths have a wingspan of about 1.5 inches. The adult moth lays her eggs on the underside of leaves in clusters of a couple hundred.

Regarding control, it is recommended that in most cases you do nothing except remove parts of branches as described above. Natural predators include birds and wasps. For severe infestations, the bacteria Bacillus thuringiensis (B.T.) can be applied to the leaves surrounding the web so the worms eat those leaves as the web expands. There is really no practical way to get at the worms while they are inside the web. Do not attempt to burn the web while it is in the tree.

Since the fall webworm infests deciduous trees, the appearance problem will be gone as the leaves fall. However, if you have an infestation in a tree, dispose of the fallen leaves by burning or putting them in the garbage instead of composting them, so that the webworm won't overwinter in the pupal stage.

Coming up: From 7 to 9 p.m. on Monday, Sept. 27, Mark Tapley of the Cascade Bonsai Society will teach a class on Bonsai, which is an ancient, artistic form of cultivating trees in pots. The class will include demonstrations of the techniques of wiring, pruning and fertilizing. Cost of the class is $5, and will be held in the auditorium of the Southern Oregon Research and Extension Center, 569 Hanley Road in Central Point. Call 541-776-7371 for more information.

Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. E-mail her at diggit1225@gmail.com.

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